The use of membranes in various forms for the separation of fluids to obtain a desired portion of the fluid is well known. Generally, one or several components of the feed fluid permeate through the membrane and are collected as the permeate portion. The portion of the fluid that cannot pass through the membrane, the retentate, is retained and discharged as fresh portions of the fluid to be separated are supplied to the membrane.
Membranes formed as hollow fibers or tubes are particularly useful because they are inherently strong to resist filtration pressures, they provide high surface area to volume ratios and, they can be readily arranged in various mechanical mountings. Conventional separation modules are configured as long cylinders with the hollow fibers arranged in an axial direction and terminated by plugs of potting material. One or both plugs are sliced to expose the open fiber ends and permit the flow of the permeate from the lumen of the tubular fiber.
In the existing devices, the fluid to be separated may be supplied to the outside of a fiber and the permeate may be collected from the lumen of the fiber. Alternatively, the fluid to be separated may be supplied to the lumen of the fiber and the permeate drained from outside of the fiber.
Hollow fiber membranes may be conveniently mounted in annular or similar frames or retainers having a continuous perimeter and an open central portion. The fibers are strung across the open central portion of the frame and the ends are embedded in the retainer thereby forming a wafer. The ends of the fibers are exposed at the outside surface of the retainer, giving access to the interior of the fibers, while the outside surfaces of the fibers are accessible in the open central portion of the retainer.
In order to obtain relatively high volume separation rates, membrane wafers are generally stacked coaxially so that the retainers bear on each other in sealing contact. Tight sealing of adjacent wafers is essential to avoid contamination of retentate and permeate. To achieve desirably high volume separation rates, the pressure of the fluid being subjected to the separation process is as large as possible. However, the fluid pressure is limited by the mechanical strength of the fibers, so that they are neither crushed nor ruptured, depending upon the direction of the pressure differential. The operating pressure is also limited by the fluid forces that tend to force the wafers apart, threatening the sealing engagement of adjacent wafer retainers.
Such membranes and corresponding separators incorporating the membranes are disclosed in various patents and applications. U.S. Pat. No. 3,993,816, for instance, describes an apparatus in which the interiors of the hollow fibers are connected to the exterior of the container so that either fluid permeate flows out of the apparatus or, in the event of fluid exchange, a second fluid flows in through the interiors of the fibers.
U.S. Pat. No. 4,752,305 provides a hollow fiber device for separating fluids and a related method. The device employs a bundle of fibers 11, oriented axially of the housing 10. The fluid feedstream is fed through the center of the bundle via distributor tube 12, one end of which is plugged. The permeate fluids flow out of each end, via outlets 39 and 40, while the retentate is drawn from outlet 35.
French Pat. No. 2,222,134 discloses a module separator wherein the wafers comprise a fabric placed in a tube perpendicular to the direction of flow so that the openings of the hollow fibers are located around the periphery.
Japanese Pat. No. 56-28031 discloses a hollow fiber membrane separator consisting of a hollow fiber tube plate formed by use of a synthetic resin which fixes and integrates the ends of the hollow tubes, and a channel for fluid flow outside of the hollow tubes. The tube plate is disposed such that the flow direction of the fluid outside of the tubes is perpendicular to the longitudinal axis of the hollow fiber tubes.
Finally, in German Pat. No. 2,650,341 a hollow fiber wafer is disclosed for use in a separator in which the hollow fibers are arranged in planes with an essentially non-parallel arrangement, essentially perpendicular to the flow direction of the substance to be separated.
While the aforementioned art discloses some of the ways in which hollow fiber wafers may be employed in a separator apparatus, it is nonetheless desirable to provide simple separation apparatus employing a plurality of hollow fiber membrane wafers in which the seals between adjacent wafers in the stack are maintained. Furthermore, it is desirable that the apparatus respond to changes in the fluid inlet pressure by altering the compression forces applied to the wafer stack to aid the sealing engagement of adjacent wafer retainers.